In a large roasting pan coated with nonstick spray, toss together popcorn, graham cracker pieces and 1 cup marshmallows.

In a large saucepan, melt together butter, remaining 5 cups marshmallows and salt, stirring until smooth. Remove from the heat and stir in vanilla. Pour melted marshmallows over the popcorn mixture and stir to coat - set aside until cool enough to handle.